The present invention relates to radio receiving apparatus of the type for receiving and processing spread spectrum radio signals. Such radio signals are commonly used in satellite communication systems, and particularly in navigation systems such as the GPS system, and in other communications systems requiring a high level of immunity to Gaussian noise and jamming, such as low probability of intercept communications systems.
The GPS system is a satellite based global passive radio navigation system which enables a properly equipped user to calculate his position to an accuracy of a few meters and his velocity to a few tenths of meters per second in three dimensions. Worldwide coverage is obtained with a network of satellites in dispersed non-geosynchronous orbits, with a minimum of four and an average of six satellites visible at all times from any point on the earth's surface. All satellites transmit signals referenced to a common system time continuously on two common frequencies around 1575 MHz (L.sub.1) and 1228 MHz (L.sub.2). The signals consist of ranging codes, unique to each satellite, which are modulated with a data stream which gives the user an accurate position of the transmitting satellite, an almanac for the whole system to enable him to choose the best satellites, and various corrections and status information. Each transmitted signal is spread over a wide band by modulation with a binary pseudo-noise (PN) (or pseudo-random) code sequence generated at a code chip frequency substantially greater than the data rate. The signal bandwidth is about 20 MHz at each frequency, and the transmitted polarization is circular. Position is found by measuring the pseudo-ranges to four satellites. These are ranges measured by estimating the propagation time using a receiving clock which is not aligned with system time. Four such measurements enable the user's position and the time offset in his clock to be calculated. Likewise, four Doppler measurements enable the velocity and clock frequency error to be found.
In prior art receivers, the received signal is demodulated by multiplying the incoming modulated signal by a coherent replica of the carrier, and low pass filtering to strip off the carrier Doppler, then multiplying by a locally generated code sequence. If the locally generated code sequence is in phase with the received code sequence, the transmitted message sequence results. Alternatively, the signal may be demodulated by multiplying by a synchronous replica of the code, then removing the carrier Doppler. In either case, the signal remaining after stripping the code or carrier is quite low, and high levels of initial amplification of the received signal from the antenna are necessary to assure that the processed signal has an acceptable level.
The received signal levels at the antenna are typically 30db below thermal noise, and the ground receiver requires narrow bandwidth tracking loops to lock onto the signal. The hardware necessary to perform the required signal discrimination and amplification is complex and costly. In general, prior art receivers include a separate channel for each satellite to be tracked. Each channel includes analog circuitry for amplification, and tracking oscillators and other analog components for in-phase and quadrature determination, which introduce considerable noise into the system, and further introduce errors, due, for example, to variations or mismatching of components and processing in the different channels.